1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to handling of measurement reports in a wireless communication system.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a simplified diagram illustrating a conventional universal mobile telecommunications system (UMTS) network, which consists of at least one user equipment (UE) 2 and a UMTS terrestrial radio access network (UTRAN) 4. In general, the UE is equivalently the mobile station, and the UTRAN 4 comprises a base transceiver station (which is termed “node-B” 42 in UMTS) and a base station controller (which is termed “radio network controller” (RNC) 44 in UMTS).
Among the protocol layers of the UMTS, the radio resource control (RRC) layer plays an important role for wireless access. The RRC layer provides functions such as control of quality of service (QoS), information exchange between the UE and UTRAN, and configuration of radio resources. To configure the radio resources control, certain measurements are necessary. The RRC layer decides what to measure, when to measure it, and how to report the measurement results, including both over the UMTS air interface and other radio systems. It also constructs reports of the measurement results from the UE to the network. When the UE is in idle modes the measurements are usually internal to the UE. It is also possible that some of the measurement results must be reported to the network when the UE is in connected mode. The descriptions of RRC layer are defined in specification “3GPP TS 25.331 Radio Resource Control (RRC)” and hence are not repeated in detail.
Besides, there are four RRC states defined in the connected mode: CELL_DCH state, CELL_FACH state, CELL_PCH state and URA_PCH state. These states reflect the level of UE connection and the transport channels that can be used by the UE. For example, the CELL_PCH state and URA_PCH state are characterized by the fact that there is no uplink activity, while in the CELL_DCH state a dedicated physical channel is assigned; though no dedicated physical channel is allocated in the CELL_FACH state, a default common or shared transport channel is assigned. To make the UE to have an RRC state transition, the UTRAN can initiate any one of the following radio bearer control procedures, which is triggered by the UE's reception of a radio bearer control message from the UTRAN and the UE responds to the message with a corresponding radio bearer control complete message:
1. radio bearer setup procedure;
2. radio bearer reconfiguration procedure;
3. radio bearer release procedure;
4. transport channel reconfiguration procedure; and
5. physical channel reconfiguration procedure.
Upon receiving a radio link control (RLC) acknowledgement message from the UTRAN, the UE transits to a new RRC state and hence completes the RRC state transition.
To perform the measurements in connected mode, the UTRAN must first send a measurement control message to the UE. Within the measurement control message, there are several information elements (IEs) relating to what kind of measurement procedure should be performed. For example, the IE “measurement command” can have the value of “set up”, “modify” or “release”, and the IE “measurement type” specifies the types of measurement(s) the UTRAN wants the UE to perform. The UE measurements can be categorized into seven types as below:
1. Intra-frequency measurements;
2. Inter-frequency measurements;
3. Inter-RAT measurements;
4. UE positioning measurements;
5. Traffic volume measurements (TVM);
6. Quality measurements; and
7. UE internal measurements.
The results of the measurements are sent back to the UTRAN in measurement report messages either periodically, after some triggering event, or using a combination of the two. There is a long list of events that may trigger a measurement report, but a UE does not normally have to report all these events; there is a pool of events from which the UTRAN can choose the events to be reported. For example, the first report is sent after a triggering event, and the following messages are sent periodically, while the same triggering conditions still apply.
Please refer to FIG. 2. FIG. 2 illustrates a message sequence chart of a TVM measurement report procedure in normal condition:
Step 110: The UE is originally configured to be in the CELL_DCH state or the CELL_FACH state;
Step 112: The event 4b of the measurement type of “traffic volume” measurement is triggered;
Step 114: The UE sends a measurement report message (MEASUREMENT REPORT message in 3G specification is sent in this example) to the UTRAN;
Step 116: The UTRAN determines to make the UE enter a different state such as the CELL_FACH state (when the UE is originally in CELL_DCH state), the CELL_PCH state or the URA_PCH state (in this example, the CELL_PCH state is entered);
Step 118: The UE receives a radio bearer control message from the UTRAN (RADIO BEARER RECONFIGURATION message in 3G specification is sent in this example);
Step 120: The UE transmits a corresponding radio bearer control complete message to the UTRAN (RADIO BEARER RECONFIGURATION COMPLETE message in 3G specification is sent in this example);
Step 122: The UTRAN sends an RLC acknowledgement to confirm the reception of the radio bearer control complete message from the UE; and
Step 124: The UE enters a new RRC state due to the radio bearer control message in Step 118 (in this scenario, the CELL_PCH state is entered).
There may be various reasons prompting the UTRAN to command the UE to enter CELL_FACH state, CELL_PCH state or URA_PCH state. According to this example, the UE sends a measurement report message in Step 114 notifying the UTRAN of the event 4b, which indicates that transport channel traffic volume becomes smaller than an absolute threshold. In other words, the UE has no more data to be sent in the uplink channel, and it can be readily appreciated that the UTRAN is allowed to make the UE enter the CELL_FACH, CELL_PCH state or the URA_PCH state. As described above, the UTRAN hence initiates the radio reconfiguration procedure by sending a RADIO BEARER RECONFIGURATION message in Step 118, and the UE responds with a RADIO BEARER RECONFIGURATION COMPLETE message in Step 120. After receiving an RLC acknowledgement message from the UTRAN, the UE enters a new RRC state, as shown in Step 122 and 124.
However, some measurement reports are unnecessary under certain scenarios. Please refer to FIG. 3. FIG. 3 illustrates one scenario in which some problems could occur:
Step 210: The UE is originally configured to be in the CELL_DCH state or the CELL_FACH state;
Step 212: The event 4b of the measurement type of “traffic volume” measurement is triggered;
Step 214: The UE sends a measurement report message (MEASUREMENT REPORT message in 3G specification is sent in this example) to the UTRAN;
Step 216: The UTRAN determines to make the UE enter a different state such as the CELL_FACH state (when the UE is originally in CELL_DCH state), the CELL_PCH state or the URA_PCH state (in this scenario, the CELL_PCH state is entered);
Step 218: The UE receives a radio bearer control message from the UTRAN (RADIO BEARER RECONFIGURATION message in 3G specification is sent in this example);
Step 220: The UE transmits a corresponding radio bearer control complete message to the UTRAN (RADIO BEARER RECONFIGURATION COMPLETE message in 3G specification is sent in this example);
Step 222: Another event is triggered;
Step 224: The UTRAN sends a radio link control (RLC) acknowledgement to confirm the reception of the radio bearer control complete message from the UE;
Step 226: The UE enters a new RRC state due to the radio bearer control message in Step 218 (in this scenario, the CELL_PCH state is entered); and
Step 228: The UE enters CELL_FACH state because a measurement report must be sent due to Step 222;
Within this scenario, an event occurred between a RADIO BEARER RECONFIGURATION COMPLETE message sent in Step 218 and an RLC acknowledgement message sent in Step 220. That is possible because there could be a time interval, say 150 ms, between Steps 218 and 220. If any event is triggered during this time interval, the UE needs to enter CELL_FACH state immediately to send the corresponding measurement report. Since the UTRAN has commanded the UE enter the CELL_PCH state, as shown in Step 216, the UE is not supposed to have any uplink activity. Therefore, this measurement report becomes unnecessary. These unnecessary measurement reports may have the measurement type of traffic volume (the event 4b is triggered), intra-frequency, inter-frequency, inter-RAT, quality, or UE internal measurement. This not only wastes the UTRAN resource due to CELL_FACH reconfiguration but also consumes UE power due to unnecessary measurement transmission in CELL_FACH state.